Currently, lots of different wireless communication networks coexist, such as Global System for Mobile communication (GSM)/Code Division Multiple Access (CDMA)/Long Term Evolution (LTE)/Wireless Fidelity (WiFi)/Worldwide Interoperability for Microwave Access (WiMAX) and so on. In many areas in the communication industry, there are still some legacy analog wireless systems, which occupied the best frequency bands but with very low spectral efficiency.
Since these legacy wireless systems usually have a very narrow frequency bandwidth and the data rates are also very low, it is possible and valuable to build a new wide band system to share spectra of these legacy wireless systems. However, for the new wide band system, all in-band signals coming from legacy heterogeneous systems would be treated as interference signals.
Most of the narrowband signals are very strong in terms of power spectrum density, and traditional wireless systems do not work well because of lacking interference resistance capabilities. In particular, with the presence of strong interferences from a heterogeneous network on a synchronization channel, the synchronization of a wide band system might be greatly impacted, thereby reducing the system performance. Therefore, it will be a big challenge to make the wide band signal to coexist with the strong legacy signal.